Surface coating of ferrous materials

Abstract

The invention relates to a method for coating the surface of an iron-containing material / base body with an iron oxide layer. The invention also relates to an iron-containing base body coated by this method, and to a device for carrying out the method according to the invention.

Description

[0001] The invention relates to a method for coating the surface of a ferrous material / base body with an iron oxide layer. The invention also relates to a ferrous base body coated by this method, as well as to a device for carrying out the method according to the invention.

[0002] The coating of ferrous base materials, such as tools used in hot metal processing, is already known in the art. For example, to produce a seamless tube, a hot steel block is rolled over a piercing mandrel in a cross-rolling process. During this process, which typically lasts between 5 and 30 seconds, the tool, i.e., the piercing mandrel, is subjected to high stress.

[0003] To increase the service life of the piercing mandrel, it is also known to coat it with an iron oxide layer (also known as scale). In other words, an iron oxide layer / scale layer is deliberately created on the base of the piercing mandrel, i.e., a so-called forced scaling process is carried out.

[0004] It is also known in the prior art that such forced scaling of an iron-containing base body can be achieved by contacting the base body with an oxidizing agent and subsequently heating the base body and oxidizing agent.

[0005] The quality / stability of the iron oxide layer produced depends largely on the process used to create it on the iron-containing base body.

[0006] The object of the invention is to provide an improved manufacturing process by which an iron oxide layer can be produced on the surface of an iron-containing base body. In particular, the process according to the invention leads to improved adhesion of the iron oxide layer to the base body, thereby at least reducing flaking of the iron oxide layer during the coating process itself and during subsequent use of the coated tool. This significantly increases the service life of the tool surface-coated with an iron oxide layer according to the invention.

[0007] The object underlying the invention is achieved by a method having the features of claim 1, an iron-containing base body coated according to the invention having the features of claim 21, and a device for coating according to the invention having the features of claim 24. Advantageous embodiments are described in the dependent claims.

[0008] More specifically, the invention relates to a method for producing at least one iron-containing base body, in particular a tool, with an iron oxide layer with a layer thickness of ≥ 20 µm and ≤ 3000 µm. The method comprises at least the following steps: a) heating the at least one base body by means of a heating device to a temperature > 500 °C; b) introducing an oxidizing agent into the heating device for contact with the iron-containing base body, so that an iron oxide layer forms on the surface of the base body; and c) cooling the iron oxide-coated base body.

[0009] The invention is based, among other things, on the finding that the time at which the oxidizing agent comes into contact with the iron-containing substrate significantly influences the quality of the resulting surface-coated substrate. In particular, it was shown that contacting the substrate with the oxidizing agent only at a time when the substrate had already been heated to a temperature above 500 °C led to a significant improvement in the process. In particular, this resulted in a significantly reduced or even completely absent formation of chromite spinels and / or chromite layers.

[0010] The iron-containing base body can be any suitable iron-containing base body, in particular a base body of a tool and / or an iron-containing base body comprising chromium.

[0011] The tool can, in particular, be a tool for use in hot metal processing. Particularly preferred tools include a piercing mandrel or a plug, both particularly for seamless tube production, a rod, particularly for use as an internal tool, a guide shoe or a Diescher disc, both particularly for use on a cross-roller, or a roll, particularly for use as an external roll. According to the invention, a piercing mandrel is preferred as a tool because it has high iron oxide layer thicknesses and a high quality of forced scaling leads to a significant improvement in service life.

[0012] The process according to the invention further leads to the formation of an iron oxide layer on the surface of the iron-containing base body, wherein the iron oxide layer has a layer thickness in the range of ≥ 20 µm and ≤ 3000 µm. The formed iron oxide layer can comprise FeO, Fe3O4, and / or Fe2O3.

[0013] In step a) of the method according to the invention, the at least one base body is heated to a temperature of > 500°C by means of a heating device. This temperature can be referred to as the minimum temperature. The base body is preferably arranged within the heating device. The heating device can be designed as a furnace and / or have heating elements, with a bell furnace or a shaft furnace being particularly preferred. The heating elements can be arranged within the wall and / or on the inside of the wall of the heating device. The heating device can be heated electrically and / or by gas firing. The heating device can be configured to heat the iron-containing base body via radiant heat and / or convection. An electrically heated heating device may or may not include induction heating.Induction heaters have the advantage of faster and more even heating of the iron-containing base body.

[0014] In step b) of the method according to the invention, only after the base body has been heated to a minimum temperature is the oxidizing agent introduced into the heating device to contact the heated, iron-containing base body. By contacting the heated, iron-containing base body with the introduced oxidizing agent, the desired iron oxide layer with a layer thickness of ≥ 20 µm and ≤ 3000 µm forms on the surface of the base body. Step a) of the method according to the invention can also be continued beyond the time of step b). In other words, the base body can also be further heated by means of the heating device beyond step a).

[0015] The heating device can have a retort that is arranged within or in the heated volume of the heating device and is suitable or configured to receive at least one, preferably a plurality of, iron-containing base bodies. Particularly preferably, the retort can have a frame that is arranged within or in the volume of the retort and is suitable or configured to receive one, preferably a plurality of, iron-containing base bodies. The frame can have at least one level for receiving at least one, preferably a plurality of, base bodies. The frame preferably has a plurality, preferably two, three, four or ≥ 5 such levels. The levels of the frame can be arranged one above the other, in particular along the vertical extent of the heating device and / or along the vertical extent of the retort.The levels of the frame can be connected to one another via at least one, preferably at least one centrally arranged, connecting element. The connecting element can be configured for anchoring and / or inserting the frame into the retort, in particular via a coupling element arranged at the end. The connecting element can have a handle area for removing the frame from the retort. This handle area can be arranged, in particular, at the end and opposite the coupling element.

[0016] The oxidizing agent can be introduced into the retort, i.e., into the interior or volume of the retort. The retort can be a gas-tight retort. Further details regarding the design of the retort are provided below and with reference to the device according to the invention.

[0017] While according to the invention any suitable oxidizing agent can be used to produce an iron oxide layer on an iron-containing base body, the oxidizing agent is preferably selected from O2, CO2, water vapor, and mixtures thereof.

[0018] The oxidizing agent is preferably introduced continuously into the heating device. This has the advantage, among other things, that a sufficient amount of oxidizing agent is available at all times for the formation of the iron oxide layer and that the contact time can be reduced.

[0019] In step c) of the method according to the invention, the now iron oxide-coated base body is cooled. Cooling preferably refers to active cooling of the iron oxide-coated base body, although passive cooling is also possible. In other words, during active cooling, the iron oxide-coated base body is not merely cooled to room temperature, but is cooled, for example, using a cooling device and / or a gas. Furthermore, the introduction of the oxidizing agent into the heating device is preferably terminated before method step c).

[0020] The base body can comprise or consist of any suitable ferrous material; preferably, the material is steel or hot-work steel. The chromium content of the steel or steel alloy is preferably as low as possible, advantageously further reducing the formation of chromite. The ferrous material of the base body can preferably have a chromium content of ≤ 4 wt.%, ≤ 2.0 wt.%, or ≤ 1.0 wt.%.

[0021] Particularly good coating results were achieved with a steel whose chromium content is in a range of ≥ 0.60 wt% and ≤ 1.55 wt%, preferably ≥ 0.90 wt% and ≤ 1.45 wt%.

[0022] It was further shown that advantageous coating results, in particular piercing mandrels with increased service life, could also be achieved if, alternatively or in addition to the chromium content according to the invention, the steel contained the smallest possible amounts of sulfur and / or phosphorus. The sulfur content is particularly preferably ≤ 0.035 wt.%, preferably in a range ≥ 0 wt.% and ≤ 0.035 wt.%, or ≥ 0 wt.% and ≤ 0.01 wt.%. The phosphorus content is particularly preferably ≤ 0.035 wt.%, preferably in a range ≥ 0 wt.% and ≤ 0.035 wt.%, or ≥ 0 wt.% and ≤ 0.01 wt.%. Alternatively or additionally, advantageous coating results, in particular piercing mandrels with increased service life, could be achieved if the manganese content in the steel is in a range ≥ 0.20 wt% and ≤ 0.45 wt%, or ≥ 0.35 wt% and ≤ 0.40 wt%.

[0023] Alternatively or additionally, the steel contains tungsten. Preferably, the steel has a tungsten content of ≥ 0.5 wt.%, ≥ 1.7 wt.%, or ≥ 3.9 wt.%. Particularly preferably, the tungsten content of the steel is in a range of ≥ 4.00 wt.% and ≤ 9.10 wt.%, preferably ≥ 7.90 wt.% and ≤ 8.20 wt.%. Such tungsten-containing coated tools exhibit an increased service life.

[0024] Even more preferably, the steel is material 1.2740 or 1.2581 (according to DIN EN 10027-2).

[0025] The base body preferably has a surface with high roughness, which advantageously achieves better adhesion of the iron oxide layer to the base body. According to the invention, the step of enlarging the surface of the iron-containing base body can be carried out before process step a). The surface of the iron-containing base body can preferably be enlarged mechanically, in particular by a subtractive manufacturing process, such as blasting or the introduction of grooves. Particularly good results have been achieved using a blasting process, in particular using a sharp-edged blasting material, preferably sand or sharp-edged steel shot.

[0026] The roughness of the surface of the ferrous base body can be determined by the average roughness depth (R z ) and / or the mean roughness value (R a ) - both according to DIN 4768, in particular DIN 4768:1990-05. Preferably, Rz a value ≥ 4 µm, more preferably ≥ 10 µm, ≥ 16 µm, ≥ 25 µm, ≥ 40 µm, ≥ 63 µm, ≥ 100 µm or ≥ 160 µm. Preferably, R a a value ≥ 0.8 µm, more preferably ≥ 1.6 µm, ≥ 3.2 µm, ≥ 6.3 µm, ≥ 12.5 µm, ≥ 25 µm, or ≥ 50 µm. More preferably, R z a value ≥ 100 µm and R a a value ≥ 25 µm or R z a value ≥ 160 µm and R a a value ≥ 50 µm. This applies particularly to the two previously mentioned materials 1.2740 or 1.2581.

[0027] The roughness of the iron-containing base body also refers to an iron-containing base body that already has an iron oxide layer according to the invention on its surface, i.e., the result of the coating process according to the invention. In this case, the roughness does not refer to the surface of the iron oxide layer of the coated base body, but to the surface of the base body located beneath this iron oxide layer, i.e., the uncoated base body.

[0028] The iron oxide layer formed on the base body by means of the method according to the invention can have a chromite content which is ≤ 0.5% by weight or ≤ 0.3% by weight.

[0029] The iron oxide layer formed on the base body by means of the method according to the invention can have a thermal conductivity (λ) that is ≤ 20% of the thermal conductivity of the iron-containing base body. This advantageously reduces the thermal load on the base body of a tool, thus increasing the service life. The formed iron oxide layer can have a thermal conductivity that is ≤ 5.0 W / (m*K), ≤ 4.0 W / (m*K), or ≤ 1.0 W / (m*K), or in a range of ≥ 0.8 W / (m*K) and ≤ 5.0 W / (m*K), preferably ≥ 1.0 W / (m*K) and ≤ 4.0 W / (m*K).

[0030] In a preferred embodiment, the base body is heated during process step a) to essentially the optimum temperature for the coating process. This temperature is always above 500°C. The base body can be heated during process step a) to a temperature that is ≥ 850°C, ≥ 900°C, or ≥ 950°C, or that lies in a range ≥ 800°C and ≤ 1050°C, preferably ≥ 850°C and ≤ 1000°C, and more preferably ≥ 925°C and ≤ 975°C. It has been shown that the formation of the iron oxide layer was significantly accelerated at these temperatures. Advantageously, this can shorten the duration of contact between the base body and the oxidizing agent and / or shift the reaction equilibrium toward the formation of the iron oxide layer.

[0031] Alternatively or additionally, the contacting of the base body and oxidizing agent in process step b) can take place over a period of ≥ 3 h or ≥ 4 h, and / or in a range of ≥ 3 h and ≤ 24 h, preferably ≥ 3 h and ≤ 9 h, or ≥ 4 h and ≤ 8 h. Particularly good results were achieved by combining heating of the base body in a temperature range of ≥ 850 °C and ≤ 1000 °C or ≥ 925 °C and ≤ 975 °C with a contacting period in the range of ≥ 3 h and ≤ 9 h or ≥ 4 h and ≤ 8 h. In particular, layer thicknesses of the iron oxide layer of ≥ 300 µm and ≤ 900 µm could be achieved.

[0032] The iron oxide layer produced on the base body by the method according to the invention has a layer thickness in the range of ≥ 20 µm and ≤ 3000 µm. Preferably, the layer thickness is in a range of ≥ 200 µm and ≤ 1000 µm, even more preferably in a range of ≥ 300 µm and ≤ 900 µm. These layer thicknesses have proven particularly suitable for the production of piercing mandrels, especially in combination with the aforementioned materials 1.2740 or 1.2581.

[0033] In a further preferred embodiment, the oxidizing agent is a gas mixture, in particular a gas mixture consisting of water vapor and O2, wherein the gas mixture has an O2 content in the range of ≥ 5 vol.% and ≤ 25 vol.%, preferably ≥ 11 vol.% and ≤ 21 vol.%. Particularly in conjunction with a bell furnace or a shaft furnace as the heating device, an improvement in the homogenization of the temperature distribution within the heating devices could be achieved by increasing the O2 content to the range according to the invention. The improvement in the homogenization of the temperature distribution occurred in particular when a gas mixture of water vapor and O2 was present. As a result, an improved iron oxide-coated product was also obtained.

[0034] Additionally or alternatively, the oxidizing agent can be introduced into the heating device at a temperature in the range of ≥ 130 °C and ≤ 200 °C, preferably ≥ 150 °C and ≤ 180 °C. Once again, an improvement in the coating of the base body could be achieved by maintaining these temperature ranges. The stated temperature ranges at which the oxidizing agent is introduced into the heating device can be the production temperature of the oxidizing agent.

[0035] As will be described in more detail later in the application, the oxidizing agent can be introduced into the heating device, in particular the retort, at a position below the iron-containing base body to be coated, which is arranged at the bottom.

[0036] In particular, if the retort is a gas-tight retort, it may have a lid. The lid of the retort may further have at least one seal. At least during process step b), at least a portion of the lid, preferably the at least one seal of the lid, may be cooled.

[0037] In a preferred embodiment of the invention, the heating device is flushed with a fluid, in particular a gas, before and / or during process step a) and / or before and / or during process step c), so that oxygen is displaced from the heating device. In other words, this ensures, on the one hand, that the iron-containing base body does not come into contact with the oxidizing agent / oxygen until the base body has been heated to an optimal temperature for the coating process in process step a). On the other hand, it is ensured that the coated base body also does not come into contact with the oxidizing agent / oxygen before it cools below the optimal temperature. This advantageously significantly reduces the formation of chromite. When reference is made to flushing the heating device, this should also include flushing of any retort arranged within the heating device.

[0038] The gas may in particular be an inert gas, preferably nitrogen.

[0039] According to the invention, the heating of the heating device for heating the base body in method step a) can take place in at least two stages, wherein the heating rate in the first stage is greater than the heating rate in the subsequent second stage. This advantageously achieves more uniform heating of the base body in the heating device. The heating rate in the first stage can be at least twice as high or at least three times as high as the heating rate in the second stage. Preferably, the heating device is heated in the first stage at a rate in a range of ≥ 15 °C / min and ≤ 20 °C / min. Likewise preferably, the heating device is heated in the second stage at a rate in a range of ≥ 2 °C / min and ≤ 5 °C / min.After heating the heating device to the target temperature, its temperature can be maintained for ≥ 15 min and ≤ 60 min before introducing the oxidizing agent, thereby ensuring that the base body to be heated also has the target temperature.

[0040] The cooling of the iron oxide-coated base body in process step c) preferably takes place to a temperature of ≤ 780 °C, ≤ 750 °C, ≤ 730 °C, ≤ 727 °C, or ≤ 723 °C. Particularly preferably, the cooling of the iron oxide-coated base body in process step c) takes place by introducing a fluid, in particular a gas, preferably an inert gas, preferably nitrogen. The gas can be introduced into the heating device or the retort arranged therein and come into direct contact with the base body. This achieves particularly rapid cooling and simultaneously removes the oxidizing agent from the heating device.

[0041] The cooling of the base body in process step c) can take place in at least two or preferably at least three stages, wherein the cooling rate in the first stage is greater than the cooling rate in the second and / or third stage. Preferably, the cooling rate in the first stage is at least twice or at least three times as high as the cooling rate in the second and / or third stage.

[0042] Particularly preferably, the first stage of cooling the coated base body can be carried out by introducing a gas, as described above. Further preferably, the heating device and / or the coated base body can be cooled in the first stage at a rate in a range of ≥ 23 °C / min and ≤ 11 °C / min, preferably ≥ 19 °C / min and ≤ 14 °C / min, or at 16 °C / min. In addition, the cooling of the coated base body in the second stage can be carried out by means of an air stream directed onto the retort, for example using a fan. In addition, the cooling of the coated base body in the third stage can be carried out by passive cooling, i.e. by allowing the retort to stand at room temperature.

[0043] The object underlying the invention is also achieved by an iron-containing base body having an iron oxide layer arranged thereon, wherein the base body has a chromium content of ≤ 4 wt.%, the layer thickness of the iron oxide layer is in a range of ≥ 20 µm and ≤ 3000 µm, the thermal conductivity of the iron oxide layer is ≤ 20% of the thermal conductivity of the iron-containing base body and no chromite layer and / or chromite spinels are arranged between the iron oxide layer and the base body.

[0044] The term chromite layer should also be understood to mean partial layers of chromite.

[0045] With regard to the further features of the iron oxide-coated base body provided by the method according to the invention, reference is made to the above explanations on the method.

[0046] However, an iron oxide-coated iron-containing base body having a chromium content in a range of ≥ 0.60 wt.% and ≤ 1.55 wt.% and a layer thickness of the iron oxide layer in a range of ≥ 300 µm and ≤ 900 µm is particularly preferred.

[0047] Alternatively or additionally, an iron oxide-coated iron-containing base body is also preferred, the surface of the base body having an average roughness depth (Rz) ≥ 4 µm and / or a mean roughness value (Ra) ≥ 0.8 µm.

[0048] The object underlying the invention is also achieved by a device suitable and / or configured for carrying out the method according to the invention and / or suitable or configured for providing the iron oxide-coated iron-containing base body according to the invention.

[0049] The device according to the invention comprises at least one heating device, in particular a bell furnace and / or a shaft furnace. It further comprises at least one gas-tight retort arranged in the heating device, which is designed to accommodate at least one iron-containing base body. It further comprises at least one first introduction device for introducing a gaseous oxidizing agent into the gas-tight retort.

[0050] The first introduction device may also be suitable and / or configured to introduce a further fluid, in particular an inert gas such as nitrogen, into the gas-tight retort.

[0051] Alternatively or additionally, the device may comprise a second inlet device for introducing a further fluid, in particular an inert gas, into the gas-tight retort.

[0052] The first and / or second inlet device may comprise a further heating device for heating the oxidizing agent and / or the further fluid. The first and / or second inlet device may comprise a cooling device, in particular for cooling the further fluid.

[0053] The first and / or second introduction device may comprise a pipe section for introducing the oxidizing agent and / or the further fluid.

[0054] The first and / or the second introduction device, or a part thereof, can preferably be guided through the lid of the gas-tight retort.

[0055] The retort, in particular the lid of the retort, can preferably have a passage for the outlet of a fluid, in particular a gas. The passage can have a valve, in particular a check valve. The exiting fluid can essentially be water vapor and H2.

[0056] The heating device can have at least two separately controllable heating zones along its vertical extent for heating the base body.

[0057] The device according to the invention can have a control unit which is designed to control the heating device and / or the introduction device, in particular according to method steps a), b) and / or c).

[0058] In particular, the control unit can be configured to control the heating device, in particular its various heating zones, to heat the base body to a temperature of > 500°C. Alternatively or additionally, the control unit can be configured to control the introduction of the oxidizing agent and / or another fluid via the first introduction device. Alternatively or additionally, the control unit can be configured to control the introduction of another fluid via the second introduction device. Alternatively or additionally, the control unit can be configured to control the cooling of the coated base body via the introduction of the another fluid.

[0059] The control unit may further comprise a timer for determining and / or controlling contact between the base body and the oxidizing agent. The timer may be configured to ensure contact between the base body and the oxidizing agent within the previously specified time periods.

[0060] With regard to the further features of the device according to the invention, reference is also made to the above explanations of the method.

[0061] Further advantages, details and features of the invention will become apparent from the following exemplary embodiments.

[0062] In detail: Fig. 1: a section through a device according to the invention along the vertical extent thereof.

[0063] The Fig.The device 1 according to the invention shown in Figure 1 is designed to carry out the methods according to the invention and / or to provide the iron-containing base body according to the invention.

[0064] The device 1 comprises a heating device 2 in the form of a shaft furnace. Several heating elements 23 can be seen, which are arranged on the inside of the path of the heating device 2. The heating elements 23 form two separately controllable heating zones 21, 22, which are arranged one above the other along the vertical extent of the heating device 2. The heating device 2 further comprises a volume 24, which can also be referred to as the interior or interior of the heating device 2.

[0065] A gas-tight retort 3 is arranged in the volume 24 of the heating device 2. The gas-tight retort 3 has a lid 31, which seals the retort in a gas-tight manner via the seal 33. Also visible is the cooling device 32 of the lid 31 in the form of a circumferential cooling channel. A cooling fluid flows through this channel, particularly during operation of the heating device, to cool the lid 31 and the seal 33.

[0066] Arranged within the retort 3 is a frame 34, which is designed to accommodate a plurality of ferrous base bodies 4. The frame 34 has three levels 35, each serving as a workpiece support for a plurality of ferrous base bodies 4. The levels 35 of the frame 34 are arranged one above the other and along the vertical extent of the heating device 2 and along the vertical extent of the retort 3, and are connected to one another via a centrally arranged connecting element 36. The frame 34 is inserted into the retort 34 via a coupling element arranged at the end and not shown in detail.

[0067] By way of example, five ferrous base bodies 4 in the form of a tool, namely a piercing mandrel, are also shown on the middle level 35 of the frame 34. In one embodiment, the ferrous base bodies 4 are made of steel, in particular the material 1.2740 (according to DIN EN 10027-2). In another embodiment, the ferrous base bodies 4 are made of steel, in particular the material 1.2581 (according to DIN EN 10027-2). Of course, when using the device according to the invention, all levels 35 of the frame 34 can be equipped with the maximum possible number of ferrous base bodies 4.

[0068] Also visible is an inlet device 5, or a part of the inlet device in the form of a pipe section.

[0069] The inlet device 5 is configured to introduce the oxidizing agent and / or another fluid, in particular nitrogen, into the retort 3. For this purpose, the illustrated pipe section of the inlet device is passed through the lid 31 of the retort 3. The pipe section of the inlet device 5 is arranged in the retort 3 such that its outlet opening is positioned below the lowest level 35 of the frame 34.

[0070] Also part of the introduction device 5, but not shown, is a further heating device which is designed to heat the oxidizing agent.

[0071] As can be seen, the lid 31 of the retort 3 has a fluid passage 37 within which a check valve is arranged. The fluid exiting through this fluid outlet 37 during operation of the device according to the invention is essentially water vapor and hydrogen.

[0072] Also part of the device 1 according to the invention, but not shown, is a control unit. This control unit is configured to control the heating device 2 and the introduction device 5. In particular, this control unit is configured to carry out the steps of the method according to the invention described below by controlling the heating device 2 and the introduction device 5.

[0073] To carry out the method according to the invention, the previously described piercing mandrels 4 (made of either material 1.2740 or material 1.2581) were positioned on the levels 35 of the frame 34 and inserted into the retort 3. The retort 3 was then closed by attaching the lid 31 and in turn inserted into the shaft furnace 2.

[0074] The inert gas nitrogen is then introduced into the retort 3 (and thus into the heating device 2) via the introduction device 5 to ensure that the piercing mandrels 4 to be coated are (initially) no longer in contact with oxygen. The control unit of the device can be configured to control the introduction of the nitrogen via the introduction device 5.

[0075] Thereafter, the piercing mandrels 4 arranged in the heating device 2 are heated by the same to a temperature above 500 °C. The control unit of the device can be configured to control the heating and / or the two separately controllable heating zones 21, 22.

[0076] The heating of the heating device 2—and thus of the piercing mandrels 4—occurs in a two-stage process, which can also be controlled by the appropriately configured control unit. In the first stage, the heating device is heated at a rate in a range of ≥ 15 °C / min and ≤ 20 °C / min and to a temperature of ≥ 750 °C and ≤ 850 °C. In the second stage, heating occurs at a rate in a range of ≥ 2 °C / min and ≤ 5 °C / min and to a target temperature of > 850 °C and ≤ 1000 °C, preferably ≥ 925 °C and ≤ 975 °C.

[0077] With the start of heating, the lid 31 and / or the seal 33 of the retort 3 can be cooled by means of a cooling fluid flowing through the cooling channel 32.

[0078] Only after heating to the target temperature has been completed, according to the invention, is the oxidizing agent introduced into the heating device 2 or the gas-tight retort 3 via the pipe section of the introduction device 5. This brings heated piercing mandrels 4 into contact with the oxidizing agent, and the formation of an iron oxide layer on the surface of the piercing mandrels 4 begins. During the contact phase between the oxidizing agent and the piercing mandrels 4, the oxidizing agent is continuously introduced into the retort 3, whereby water vapor and H2 (as a reaction product of the oxidation reaction) can escape from the retort 3 via the fluid passage 37. The control unit of the device can be configured to control the introduction of the oxidizing agent via the introduction device 5.

[0079] The introduced oxidizing agent is a gas mixture consisting of water vapor and O2, with the gas mixture having an O2 content in the range of ≥ 5 vol% and ≤ 25 vol%. In another embodiment, the gas mixture has an O2 content in the range of ≥ 11 vol% and ≤ 21 vol%.

[0080] By means of the aforementioned additional heating device of the introduction device 5 (not shown), the oxidizing agent is heated and introduced into the retort 3 at a temperature in the range of ≥ 130 °C and ≤ 200 °C, preferably ≥ 150 °C and ≤ 180 °C. The control unit of the device can be configured to control the additional heating device of the introduction device in this regard.

[0081] The contacting of the piercing mandrels 4 with continuously introduced oxidizing agent takes place over a period of time in the range of ≥ 4 h and ≤ 8 h. For this purpose, the control unit can have a timer for determining the contacting time and be configured to monitor the contacting time.

[0082] After the contact time has elapsed, the now iron oxide-coated piercing mandrels 4 according to the invention are cooled down. They have an iron oxide layer with a layer thickness in the range of ≥ 20 µm and ≤ 3000 µm. In a further embodiment—particularly when using the aforementioned materials 1.2740 or 1.258—the iron oxide-coated piercing mandrels 4 according to the invention have a layer thickness in the range of ≥ 300 µm and ≤ 900 µm.

[0083] The cooling of the iron oxide coated piercing mandrels 4 takes place in three stages, whereby the cooling rate in the first stage is greater than the cooling rate in the second and third stages.

[0084] For the purpose of active cooling, the first step involves introducing the inert gas nitrogen into the retort 3 via the inlet device 5. This ensures, on the one hand, that faster cooling is achieved. On the other hand, the oxidation process is terminated as quickly and precisely as possible. The cooling of the iron oxide-coated piercing mandrels 4 can preferably take place to a temperature ≤ 727 °C or ≤ 723 °C. The control unit of the device can be configured to control the cooling via the control of the inlet device.

[0085] For the second active cooling step, the retort 3 is removed from the shaft furnace 2 and cooled to a temperature of ≤ 500 °C by means of an air stream directed onto the retort 3.

[0086] This is followed by the third cooling step, in which the retort is cooled by passive cooling at room temperature to a temperature in the range of ≥ 50 °C and ≤ 100 °C.

[0087] After this, the iron oxide-coated piercing mandrels 4 according to the invention can be removed from the retort 3. These are characterized by significantly increased service life. List of reference symbols 1 Device suitable and / or arranged to carry out a method according to claims 1 to 20 and / or to provide an iron-containing base body according to claims 21 to 23 2 heating device; shaft furnace 21 first heating zone of the heating device 22 second heating zone of the heating device 23 Heating element 24 Volume of the heating device 3 Retort; gas-tight retort 31 Retort lid 32 Cooling device of the lid; cooling channel 33 Seal 34 frame 35 Level of the frame 36 Connecting element; central connecting element 37 Fluid passage with valve, especially check valve 4 iron-containing base body; piercing mandrel; iron oxide-coated base body; iron oxide-coated piercing mandrel 5 Part of the inlet device; pipe section QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited non-patent literature

[0000] DIN EN 10027-2 [0024, 0067] DIN 4768

[0026] DIN 4768:1990-05

[0026]

Claims

[1] Method for producing at least one iron-containing base body with an iron oxide layer with a layer thickness of ≥ 20 µm and ≤ 3000 µm, comprising the steps: a) heating the at least one iron-containing base body (4) by means of a heating device (2) to a temperature > 500 °C, b) introducing an oxidizing agent into the heating device (2) for contact with the heated iron-containing base body (4) so that an iron oxide layer forms on the surface of the base body (4), c) Cooling of the iron oxide coated base body (4). [2] Method according to claim 1, wherein the introduction of the oxidizing agent into the heating device (2) is terminated before step c). [3] Method according to claim 1 or 2, wherein the base body (4) is a tool, in particular a tool for use in hot metal processing, and wherein the tool is heated to a temperature of 950 °C in step a) and wherein the oxidizing agent is water vapor. [4] Method according to claim 3, wherein the base body (4) is selected from a piercing mandrel, in particular for seamless pipe production; a plug, in particular for seamless pipe production; a rod, in particular for use as an internal tool; a guide shoe, in particular for use on a cross-roller; a Diescher disc, in particular for use on a cross-roller; or a roller, in particular for use as an external tool. [5] Method according to one of the preceding claims, wherein the thermal conductivity of the formed iron oxide layer is ≤ 20% of the thermal conductivity of the iron-containing base body (4). [6] Method according to one of the preceding claims, wherein the iron-containing base body (4) comprises and / or consists of a steel, in particular with a chromium content ≤ 4 wt.%. [7] Method according to one of the preceding claims, wherein the surface of the base body (4) has an average roughness depth (R z ) ≥ 4 µm and / or a mean roughness (R a ) ≥ 0.8 µm. [8] Method according to one of the preceding claims, wherein in step a) the base body (4) is heated to an optimum temperature for the coating process and / or to a temperature in the range of ≥ 800 °C and ≤ 1050 °C, preferably ≥ 850 °C and ≤ 1000 °C. [9] Method according to one of the preceding claims, wherein in step b) the contacting of the base body (4) and the oxidizing agent takes place over a period of ≥ 3 h, in particular in a range of ≥ 3 h and ≤ 24 h, preferably ≥ 3 h and ≤ 9 h, or ≥ 4 h and ≤ 8 h. [10] Method according to one of the preceding claims, wherein the layer thickness of the iron oxide layer is in a range of ≥ 200 µm and ≤ 1000 µm or ≥ 300 µm and ≤ 900 µm. [11] Method according to one of the preceding claims, wherein the oxidizing agent is a gas mixture, in particular of water vapor and O2, and has an O2 content in the range of ≥ 5 vol.% and ≤ 25 vol.%. [12] A method according to any one of the preceding claims, wherein the oxidizing agent is introduced into the heating device at a temperature in the range of ≥ 130 °C and ≤ 200 °C or ≥ 150 °C and ≤ 180 °C. [13] Method according to one of the preceding claims, wherein the iron-containing base body (4) is arranged in a retort (3) arranged in the heating device (2) and the introduction of the oxidizing agent into the retort takes place at a position below the iron-containing base body arranged at the bottom. [14] Method according to claim 13, wherein the retort (3) has a lid (31) and at least a part of the lid, preferably the seal (33) of the lid, is cooled at least during step b). [15] Method according to one of the preceding claims, wherein the chromite content in the iron oxide layer formed is ≤ 0.5% by weight. [16] Method according to one of the preceding claims, wherein - before and / or during step a) and / or - before and / or during step c) the heating device (2) is flushed with a fluid, in particular an inert gas, to displace oxygen. [17] Method according to one of the preceding claims, wherein the heating device (2) for heating the base body (4) is heated in at least two stages and wherein the heating rate in the first stage is greater than the heating rate in the second stage, preferably at least twice or at least three times as great. [18] Method according to claim 17, wherein the heating device (2) is heated in the first stage at a rate in the range of ≥ 15 °C / min and ≤ 20 °C / min and / or is heated in the second stage at a rate in the range of ≥ 2 °C / min and ≤ 5 °C / min. [19] Method according to one of the preceding claims, wherein the cooling of the iron oxide-coated base body (4) takes place by introducing a gas, in particular an inert gas. [20] Method according to one of the preceding claims, wherein the cooling of the base body (4) takes place in at least two, preferably three stages and wherein the cooling rate in the first stage is greater than the cooling rate of the base body (4) in the second and / or third stage. [21] Iron-containing base body (4) having an iron oxide layer arranged thereon, characterized by , that - the base body (4) has a chromium content of ≤ 4 wt.%, - the thickness of the iron oxide layer is in a range of ≥ 20 µm and ≤ 3000 µm, - the thermal conductivity of the iron oxide layer is ≤ 20 % of the thermal conductivity of the iron-containing base body (4), and - no chromite layer and / or chromite spinels are arranged between the iron oxide layer and the base body (4). [22] Iron-containing base body (4) according to claim 21, wherein the base body (4) has a chromium content in a range of ≥ 0.60 wt.% and ≤ 1.55 wt.% and a layer thickness of the iron oxide layer in a range of ≥ 300 µm and ≤ 900 µm. [23] Iron-containing base body (4) according to claim 21 or claim 22, wherein the surface of the base body (4) has an average roughness depth (Rz) ≥ 4 µm and / or a mean roughness value (Ra) ≥ 0.8 µm. [24] Device (1) suitable and / or arranged for carrying out a method according to claims 1 to 20 and / or for providing an iron-containing base body according to claims 21 to 23, comprising: - at least one heating device (2), in particular a hood furnace and / or a shaft furnace, - at least one gas-tight retort (3) arranged in the heating device, which is designed to receive at least one iron-containing base body (4), - at least one first introduction device (5) for introducing a gaseous oxidizing agent into the gas-tight retort (3), - optionally a second inlet device for introducing a further fluid, in particular an inert gas, into the gas-tight retort. [25] Device according to claim 24, wherein the heating device has at least two separately controllable heating zones (21, 22) along its vertical extent.

Images